Daniel,
I used a bobtail curtain on 40m for many years. It was the only antenna
I have had on 40 on which I could work dx during the summer months through
the S9+ static.
One thing I did differently was to feed it at the top of the center leg
with coax, rather than using the classic parallel LC to ground. I brought
the coax off at a 45 degree angle to ground and used a choke balun at the
feedpoint. After tuning it to resonance, it matched very well to the coax
and everything agreed with the NEC model. The pattern with this method of
feed is identical to the bobtail pattern because you are feeding it at a
current node.
I made another modification which also worked out well. I noticed that
if you remove the center vertical leg and feed the horizontal part like a
dipole, it becomes a 1.5 wavelength dipole with the ends dropping to the
ground. The pattern is mostly horizontally polarized and has a strange
cloverleaf shape. I put a relay at the feedpoint and was able to switch it
between bobtail mode with the center vertical and dipole mode without it.
The patterns of those two modes were different enough so that in many
situations one mode was superior to the other. Generally, the dipole worked
better for domestic stations where the bobtail did well for dx. Then there
were those situations where the dx was high angle and the dipole's
cloverleaf pattern had favorable lobes, so it would happen to perform better
on dx. Being a large structure in either configuration, the bandwidth was
good, too.
Another advantage of the coax feed is that you can raise the entire
antenna higher off the ground to keep the ends of the verticals way up off
the ground. This way they are out of reach and the capacitive losses
between the verticals and the ground is lower.
I stumbled into using the dipole mode when I was feeding it that way while
adjusting the length of the horizontal and two end verticals. This is a
good way to tune the bobtail if you are not feeding it against ground. Tune
it in this configuration, then switch to bobtail mode and tune the center
vertical.
I did all this back in the late 80s and I always intended to write an
article on it, however, I never got around to it. Since then, I have seen
others advocating the coax feed method on the center leg and half-squares
fed at the top of an end leg. Great minds think alike, as they say.
Naturally, this design has a potential for putting current on the outside of
the coax, so a current balun is necessary. I don't remember the right
polarity for connecting the coax to the center leg and the horizontal
section. I grounded the coax where it met the ground. I modeled both
polarity cases and picked the one that had the smallest amount of current on
the outside of the feedline. I remember that there was a big difference
between the two cases and the good case had very low currents. Perhaps I
should have written that article, because then I could look it up.
Good luck with your project.
Dudley - WA1X
Message: 2
Date: Mon, 7 Feb 2005 17:47:59 -0800 (PST)
From: Daniel Hileman <n9wx_dan@yahoo.com>
Subject: [TowerTalk] Bobtail Curtain
To: towertalk@contesting.com
Message-ID: <20050208014759.16128.qmail@web52008.mail.yahoo.com>
Content-Type: text/plain; charset=us-ascii
Hi everyone,
I am thinking about putting up a bobtail curtain for 40m and hear it is an
EXCELLENT DX antenna. So, I would like some opinions on it. I already have
several designs, so I mostly just need opinions on performance and maybe any
hints and kinks that make it perform even better!
Thanks and 73,
Daniel N9WX
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